Question

How can I use a vector of thread futures to get the return values from a group of threads? Should this work? If not, how do I adapt it?

int calculate_the_answer_to_life_the_universe_and_everything()
{
    return 42;
}

vector<packaged_task<int> > tasks;
vector<unique_future<int> > futures;
for (int i=0; i < 4; ++i)
{
    tasks.push_back(packaged_task<int>(calculate_the_answer_to_life_the_universe_and_everything));
    futures.push_back(tasks.back().get_future());
    thread task(tasks.back());
}

boost::wait_for_all(futures.begin(), futures.end());

Answer 1

Well this code doesn’t compile because packaged tasks are not copyable.

Based on the problem statement, all you need is a vector of futures, though, why even try to store packaged_tasks?

#include <boost/thread.hpp>
#include <iostream>
#include <vector>
int the_answer()
{
    return 42;
}
int main()
{

    std::vector<boost::unique_future<int> > futures;
    for (int i=0; i < 4; ++i)
    {
        boost::packaged_task<int> pt(the_answer);
        futures.push_back(pt.get_future());
        boost::thread task(std::move(pt));
    }
    boost::wait_for_all(futures.begin(), futures.end());

    for(size_t n=0; n<futures.size(); ++n)
        std::cout << futures[n].get() << ' ';
    std::cout << '\n';
}